For a given airspeed, an aircraft may consume less fuel at a higher altitude than it does at a lower altitude. In other words, an aircraft may be more efficient in flight at higher altitudes as compared to lower altitudes. Moreover, bad weather and turbulence can sometimes be avoided by flying above such weather or turbulence. Thus, because of these and other potential advantages, many aircraft are designed to fly at relatively high altitudes.
As the altitude of an aircraft increases, from its take-off altitude to its “top of climb” or “cruise” altitude, the ambient atmospheric pressure outside of the aircraft decreases. Thus, unless otherwise controlled, air could leak out of the aircraft cabin causing it to decompress to an undesirably low pressure at high altitudes. If the pressure in the aircraft cabin is too low, the aircraft passengers may suffer hypoxia, which is a deficiency of oxygen concentration in human tissue. The response to hypoxia may vary from person to person, but its effects generally include drowsiness, mental fatigue, headache, nausea, euphoria, and diminished mental capacity.
Aircraft cabin pressure is often referred to in terms of “cabin altitude,” which refers to the normal atmospheric pressure existing at a certain altitude. Studies have shown that the symptoms of hypoxia may become noticeable when the cabin altitude is above the equivalent of the atmospheric pressure one would experience outside at 8,000 feet. Thus, many aircraft are equipped with a cabin pressure control system to, among other things, maintain the cabin pressure altitude to within a relatively comfortable range (e.g., at or below approximately 8,000 feet) and allow gradual changes in the cabin altitude to minimize passenger discomfort.
To accomplish the above functions, some cabin pressure control systems implement control laws that use cabin pressure rate of change as an input. In many of these systems, the cabin pressure rate of change is sensed, either using an analog or a digital pressure, and compared to a desired cabin pressure rate of change to determine a “rate error.” The rate error may then be used in the control laws to drive a motor or other actuator, which in turn moves an outflow valve to a position that causes the cabin pressure to vary at the desired cabin pressure rate of change.
Although most cabin pressure control systems that implement cabin pressure rate of change control laws operate safely, reliably, and robustly, these systems can suffer certain drawbacks. In particular, the sensed cabin pressure rate of change value can be noisy about the actual cabin pressure rate of change. For example, if sensed cabin pressure is differentiated using an analog circuit, the circuit typically includes a relatively high-gain rate amplifier that is susceptible to electrical noise. Alternatively, if the sensed cabin pressure is differentiated using software, the sensed cabin pressure is typically differentiated over relatively short time periods (e.g., 0.025 to 0.050 seconds) using relatively high-resolution (e.g., ≧19-bit resolution) analog-to-digital conversion circuits that have the same noise susceptibility as the rate amplifier.
The sensed rate noise noted above can cause the control laws to undesirably supply commands to the outflow valve motor that cause motor dither. Motor dither can cause wear on both the outflow valve motor and other components, such as gearing, that may couple to the motor to the outflow valve. Moreover, if the analog-to-digital conversion circuit is implemented as part of the cabin pressure sensor (e.g., a so-called “digital sensor”), this can result in increased sensor cost, increased transducer circuit cost, and/or relatively large sensor and circuit footprint.
Hence, there is a need for a cabin pressure control system and method that is less susceptible to circuit noise, and/or reduces or eliminates motor dither and thus motor and gear wear, and/or that can be implemented with lower cost pressure sensors and circuits. The present invention addresses one or more of these needs.